Fatty Acid Synthase Dimers Containing Catalytically Active b-Ketoacyl Synthase or Malonyl/Acetyltransferase Domains in Only One Subunit Can Support Fatty Acid Synthesis at the Acyl Carrier Protein Domains of Both Subunits*

A double-tagging, dual affinity chromatographic procedure, which permits isolation of dimers independently mutated in each subunit, has been exploited to probe the functional topology of the animal fatty acid synthase. Dimers were engineered in which the chainterminating thioesterase reaction was compromised by mutation of the (active-site) serine residue in both subunits; these dimers assembled two long-chain fatty acyl moieties, which remained covalently linked to the 4*phosphopantetheine residues of the two acyl carrier protein domains. Significantly, dimers that contained an additional mutation that compromised the activity of either the b-ketoacyl synthase or malonyl/acetyltransferase activity in only one subunit also assembled two long-chain acyl moieties. In contrast, in a control experiment, introduction of an additional mutation that compromised the function of the acyl carrier protein domain in only one subunit resulted in the assembly of only one long-chain acyl moiety per dimer. Because the b-ketoacyl synthase and malonyl/acetyltransferase domains are located near the amino terminus of the polypeptide and the acyl carrier protein domain near the carboxyl terminus, these results support a modified model for the animal fatty acid synthase in which headto-tail functional contacts are possible both within as well as between subunits.